“Lean mass increased” does not specify what changed within lean tissue. This randomized athlete study measured body compartments and performance separately. Primary report
What the trial found
Ninety-six recreationally trained adults entered an eight-week blinded trial followed by six weeks of washout. GH increased lean mass through increased extracellular water. Sprint capacity improved, while other measured performance outcomes did not significantly change. The sprint advantage was not maintained after washout. Testosterone-containing comparisons were restricted to men, and the study was too small to establish safety. Primary report
Our appraisal: laboratory capacity is not universal performance
Body-composition terminology matters. Lean mass includes more than contractile muscle. Treating every increase as new functional muscle would ignore the compartment measurements that make this experiment informative.
Performance also consists of distinct tasks. A sprint-capacity result should not become a claim about endurance, maximal strength, jumping, or real-world competitive success. The test, units, uncertainty, and practical importance should accompany any numerical estimate.
The washout observation asks whether an effect persists after exposure ends. It does not by itself identify the mechanism of loss, nor establish outcomes after longer use. It does prevent presenting the short-term finding as a demonstrated durable improvement.
Different randomization options for men and women require careful handling of pooled and sex-specific comparisons. A combined estimate cannot support every treatment combination in both sexes. Full review should retain that structure, multiplicity decisions, missing tests, and harms. This is an evidence appraisal, not guidance for performance-enhancing hormone use or a basis for assuming that research preparations reproduce the trial intervention.
Frequently asked questions
Were participants elite professional athletes?
They were recreationally trained adults.
Did increased lean mass necessarily mean more muscle tissue?
No; extracellular water contributed to the increase.
Did every performance measure improve?
No.
Did the sprint finding persist through washout?
No.
Were testosterone arms available to women?
No.
Was safety established by this sample?
No.
What is the appropriate interpretation?
A short-term, test-specific result with important compartment and persistence limits.